There are many known methods for nondestructively examining a test specimen. Many of these methods involve introducing energy into the specimen and detecting a modified form of that energy as it leaves the specimen. For example, it is known to apply X-ray, ultrasonic, magnetic, or heat energy into a test specimen and to detect flaws in the specimen as perturbations in the respective energy pattern as it returns from the specimen.
Acoustic thermography is one such form of nondestructive examination that involves the application of acoustic energy to a test specimen and the measurement of heat energy that is generated within the specimen as a result of the acoustic energy interacting with a crack. As sound energy passes through the material of a specimen, opposing surfaces of a crack or other flaw are caused to rub together, thus generating heat. Because undamaged areas of the specimen are only minimally heated by the acoustic waves, a thermal image of the specimen will reveal the flawed area as having as exhibiting an increase in temperature.
The effectiveness of an acoustic thermography examination is directly related to the efficiency of the input of acoustic energy into the test specimen. U.S. Pat. No. 6,236,049 describes the need for a coupler between the acoustic transducer the test specimen in order to couple about 30 to 40 percent of the ultrasonic energy produced by the transducer into the specimen. U.S. Pat. No. 6,399,948 describes an acoustic thermography system that delivers the acoustic energy to the specimen through an electromagnetic acoustic transducer (EMAT), thereby providing coupling to the specimen without the need for mechanical contact.